The present invention relates to a scanning grating for use in a focusing detector and to a focusing detector. The scanning grating serves as an image scanner and spatial frequency filter, and includes at least two surface elements splitting up a luminous flux into two spatially separate light fluxes and by means of which, in conjunction with a field lens, two images or parts thereof are generated in a plane in which at least two photo-receiver systems are situated.
These scanning gratings are used for the scanning of an object in the determination of the sharp focusing of an optical system.
In this regard, there is known from German Offenlegungsschrift No. 2,156,617 a "focusing detector", which is constructed as a furrow raster, which in addition to its function as a spatial frequency filter also splits up the luminous flux coming from the optical system and directs it in two different directions.
A disadvantage of this "focusing detector" is, however, the poor utilization of the light incident on it and the resulting weak signals therefrom, which can be further processed only with the application of a high degree of technical effort.
In order to be able to utilize the radiation not used in the above-mentioned "focusing detector", there has been proposed in German Offenlegungsschrift No. 2,160,877 to use a pyramid raster as a scanning grating, with which the radiation incident on it is directed to four photo-receivers. The disadvantage of using such a grating residues in the fact that the photo-receivers must be mounted so as to be associated accurately and unambiguously with the individual pupil regions. This involves large splitting-up angles, so that the partial pupils--resulting from the splitting-up at the pyramid surfaces--of the optical system are laterally separated to a sufficient extent.
In the case of large, and above all, variable pupil images, such as are inevitably generated in the case of interchangeable lenses of miniature cameras, large splitting-up angles involve problems. Besides optical defects, geometric distortions in the pupil images also arise in account of the oblique path of the incident beams. These distortions give rise to falsified signals.